In recent years, rapid development of semiconductor technology has led to rapid progress of a reduction in size of semiconductor packages, the adoption of multipin, the adoption of fine pitch, minimization of electronic components and the like. That is, the semiconductor field has entered the so-called “age of high density packaging.” Regarding printed wiring boards, this has also led to a change from single side wiring to double side wiring and, in addition, the adoption of a multilayer structure and a thickness reduction (Iwata and Harazono, “Denshi Zairyo (Electronic Material),” 35 (10), 53 (1996)).
Pattern formation methods used in the formation of such wiring and circuits include: a method which comprises the steps of: etching a metal, provided on a substrate having a layer construction of metal-insulating layer-metal, with an acidic solution, such as a ferric chloride solution, to form wirings, then subjecting the insulating layer, for example, to plasma etching, wet etching, or laser etching, to remove the insulating layer to form a desired shape, and connecting the wirings to each other, for example, through plating or electrically conductive paste; and a method (Proceedings of the 7th Symposium of Japan Institute of Electronics Packaging) which comprises the steps of: providing an insulating layer in a desired form using a photosensitive polyimide (Japanese Patent Laid-Open No. 168441/1992) or the like; and then plating gaps to form wiring.
A tendency toward downsizing of electric products in recent years has led to a reduction in thickness of each layer constituting metal conductor layer-polymeric insulating layer, and these layers each are in many cases used in a thickness of not more than 100 μm. When wiring has been formed of such thin layer, a warpage disadvantageously takes place in wiring due to a difference in coefficient of thermal expansion between the metal conductor layer and the polymeric insulating layer. Further, in the case of metal conductor layer-polymeric insulating layer-metal conductor layer, the formation of a circuit formation pattern or the like renders the area of the upper metal conductor layer different from the area of the lower metal conductor layer, and, in this case, here again a warpage takes place in wiring.
When the thermal properties of the insulating layer and the conductor layer are known, the warpage of this substrate can be calculated according to the following equation (Miyaaki and Miki, NITTO TECHNICAL REPORT, 35 (3), 1 (1997)).
  σ  =                    31        ⁢                  E          1                ⁢                  E          2                            2        ⁢                  h          ⁡                      (                                          E                1                2                            +                              14                ⁢                                  E                  1                                ⁢                                  E                  2                  2                                                      )                                ⁢    Δ    ⁢                  ⁢    α    ⁢                  ⁢    Δ    ⁢                  ⁢    T  wherein
E1: modulus of the metal,
E2: modulus of the insulating layer,
Δα: difference in coefficient of thermal expansion between the metal and the insulating layer,
ΔT: temperature difference, and
h: layer thickness 1: wiring length.
According to this equation, the following two methods are considered effective for reducing the warpage of wiring:
1. a reduction in modulus of insulating layer; and
2. a reduction in the difference in coefficient of thermal expansion between the insulating layer and the metal wiring layer.
Regarding the wiring formation method, in the substrate used in the method for the formation of wiring through etching of a metal in the laminate having layer construction of metal-insulating layer-metal or a layer construction of metal-insulating layer, in order to reduce the warpage of the substrate, a low-expansion polyimide is used as the insulating layer from the viewpoint of the necessity of rendering the coefficient of thermal expansion of the metal identical to the coefficient of thermal expansion of the insulating layer (U.S. Pat. No. 4,543,295, Japanese Patent Laid-Open Nos. 18426/1980 and 25267/1977). Since, however, the low-expansion polyimide is not generally thermoplastic, the adhesion to metals is poor making it difficult to provide adhesive strength high enough to withstand practical use. To overcome this problem, a thermoplastic polyimide resin or epoxy resin having good adhesion to the metal is used as an adhesive layer between the metal and the low-expansion polyimide.
At the present time, rapid expansion of production of personal computers has lead to increased production of hard disks incorporated in the personal computers. A component, in the hard disk, called a “suspension,” which supports a head for reading magnetism, is being shifted in its main products from one, wherein copper wiring is connected to a stainless steel plate spring, to one called a “wireless suspension” comprising copper wiring which has been connected directly to a stainless steel plate spring, from the viewpoint of coping with the size reduction.
The wireless suspension is mainly formed of a material having a three-layer structure. The material has a layer construction comprising an insulating layer, a copper alloy foil provided on one side of the insulating layer, and a stainless steel foil provided on the other side of the insulating layer. Since scanning on a disk being rotated at a high speed is carried out, fine vibration is applied to the member. Therefore, the adhesive strength of the wiring is very important. This requires satisfying severe specifications. The adhesive strength of the wiring depends greatly upon the material having a three-layer structure in its adhesive layer portion, and the ability of the adhesive layer as such determines the adhesive strength as the product.
A polyimide or similar resin, which has good insulating properties even in a thin layer thickness, is used as the resin for the insulating layer in the laminate having a layer construction of metal-insulating layer-metal or a layer construction of metal-insulating layer, particularly in the field of electronic members where long-term reliability is required. In order to impart adhesive properties to the polyimide resin, it is common practice to impart thermoplasticity. However, there is few specific studies on the relationship between the adhesive strength of the polyimide resin and the properties of adhesives. The present situation is, for example, such that, when the adhesive strength of the polyimide resin is examined, actual contact bonding followed by a peel test is necessary, that is, very troublesome work should be carried out.